Titanium fabrication

ABSTRACT

USE OF STAINLESS STEEL ENVELOPES IN HEATING AND FORGING TITANIUM BILLETS. FORMATION OF SCALE AND ALPHA CASE IS REDUCED.

United States Patent Office 3,584,368 Patented June 15, 1971 US. Cl.29424 4 Claims ABSTRACT OF THE DISCLOSURE Use of stainless steelenvelopes in heating and forging titanium billets. Formation of scaleand alpha case is reduced.

BACKGROUND OF THE INVENTION This invention relates to fabrication oftitanium. More particularly, it relates to protecting titanium billetsin such a manner that when they are heated and forged, formation ofscale and alpha case is minimized.

Titanium, that is, titanium and titanium base alloys, is prone tooxidize during heating in air to forging temperatures. This oxidationordinarily takes the form of severe surface scaling and creation of asub-surface formation called alpha-constituent or alpha case. Bothscaling and alpha-constituent are detrimental to subsequent forgingoperations and therefore undesirable. Billets of unprotected titaniumtend to scale at the rate of 0.0005 per minute when heated to forgingtemperatures in air. The term billets as used herein refers to unitarybodies made, for example, by casting, or by working such as extruding,rolling or the like. Use of an inert or non-oxidizing atmosphere is arather elaborate and expensive way to reduce scale and alpha caseformation, and glass coatings have thus far failed to reduce alpha caseand scaling to the extent desired. Therefore, providing an improvedmethod and economical alternative for minimizing scaling and alpha caseformation during titanium billet heating and forging represents a highlydesirable result.

STATEMENT OF THE INVENTION It is accordingly an object of this inventionto provide a method of minimizing scale formation and alpha-constituentformation during heating and forging cycles for titanium billets.Another object is to provide a method for improved fabrication oftitanium billets by substantially eliminating scaling and alpha caseformation. Other objects will be apparent hereinafter from thedescription and claims which follow.

My invention is predicated upon the discovery that scaling and formationof alpha case during heating and forging titanium billets is minimizedby use of austenitic stainless steel envelopes or enclosures for saidbillets. I have found that the envelopes used according to my inventionneed be no thicker than foil, that is, no greater than about .006 inchthick. If desired, lubricants, for example, a conventionalgraphite-in-oil emulsion, ma be used both on the dies used in theforging step and on the billets. Also, the titanium billets may becoated with glass frit, for example, in isopropyl alcohol, if desired,before wrapping in the stainless steel envelopes.

Sealing the envelopes after wrapping or encapsulating the titaniumbillets therein is desirable to minimize oxidation from contact withair. Sealing can be done in any convenient manner, for example, bypressing out most of the air which may be present by hand and foldingover the open end of the envelope. Another way to wrap the titaniumbillets in the envelopes and to seal the envelopes around the wrapped orencased billets is to fold a strip of foil in half and then fold andclamp the two side edges, next folding over the end several times.

According to my invention, the encapsulated billets may be heated to atleast 1600 F., for example, for up to an hour or longer, and then forgedon dies which may be maintained at a temperature of about 500 to 800 F.The austenitic stainless steel envelopes contain by weight up to 0.1%carbon, 16-20% chromium, up to 2.5% manganese and 8-13% nickel.Illustrative of such austenitic stainless steels which may be used inthe composition making up the envelopes useful according to my inventionare those listed as austenitic steels in Table 1 on page 409 of TheAmerican Society for Metals Metals Handbook, vol. 1, Properties andSelection of Metals, 8th Edition, copyright 1961.

One advantage of my invention is that the stainless steel envelopes donot have to be removed prior to forging. They may be removed during themachining operation which follows the forging or by mechanical means atany step during the fabrication process. When I refer to forging withrespect to the invention of this application, I refer to die forging aswell as to the upsetting operations.

A further understanding of my invention may be had from the examples inthe table which follows. In these examples, the titanium contained 6% byweight aluminum and 4% by weight vanadium and the billets were about 1inch in diameter by 1 inch in length. The stainless steel envelopes ofExamples 1-4, which are illustrative of my invention, containedapproximately .08% C, 2% Mn, 17- 19% Cr and 9-12% Ni. The encasedbillets were placed in a furnace and heated for an hour at 1800 F. andthen forged on fiat platen dies maintained at 650 F. on a 35- ton press.In Example 5, which is for comparative purposes to illustrate how use ofnon-stainless steel foil envelopes (this example containing 02-05% C,0.54% Mn and 0.350.8% Si) results in disintegration of the envelopes atheating and forging temperatures and formation of intolerable scale andalpha-constituent of greater than .006-inch thickness due to attack byoxygen. In Example 2, the billets were coated with glass frit prior toencasing with the austenitic stainless steel envelopes. In Examples 1and 4 a graphite in oil lubricant was used on the dies. The envelopesused in the examples were made of foil .002 inch thick. In all instancesin which austenitic stainless steel envelopes were used (Examples 1-4),there was substantially no scaling or scale formation and the alpha caseformation, as indicated, amounted to less than about .006 inch inthickness.

TABLE Alpha case thick- Example No.2 nessinches 1 .0028

1 Foil disintegnated in furnace.

From the preceding examples it is believed apparent that we haveprovided by use of austenitic stainless steel enclosures for titaniumbillets a way to substantially eliminate formation of scale and alphacase while the billets are in the furnace being heated to temperaturesof at least 1600 F. and during subsequent forging.

While the invention has been described in terms of preferredembodiments, the claims appended hereto are intended to encompass allembodiments which fall within the spirit of the invention.

Having thus described my invention and certain embodiments thereof, Iclaim:

1. A method for fabrication of titanium which comprises enclosingtitanium billets in austenitic stainless steel envelopes containing upto 0.1% by weight carbon, 16 to 20% by weight chromium, up to 2.5% byweight manganese and 8 to 13% by weight nickel, removing air from Withinthe envelopes, sealing the envelopes around the enclosed billets,heating the enclosed billets at a temperature of at least 1600 F. andthen forging the enclosed billets thereby forming a product having analpha case layer of less than about 0.006 inch in thickness.

2. The method of claim 1 wherein the envelopes sealed around theenclosed billets are of a foil thickness of not greater than 0.006 inch.

3. In a process for fabrication of titanium billets by heating andforging, the improvement which comprises encasing titanium billets inaustenitic stainless steel envelopes containing up to 0.1% by weightcarbon, 16 to 20% by weight chromium, up to 2.5% by weight manganese and8-13% by weight nickel, removing air from said envelopes having saidbillets encased therein, sealing said envelopes around the encasedbillets, heating said encased billets to a temperature of at least about1600 F., and Without removing said envelopes from said billets forgingsaid billets into shaped objects, thereby minimiz ing formation of scaleon said billets and limiting alpha 4 case formation to a layer of lessthan about 0.006 inch in thickness.

4. The improvement of claim 3 wherein the austenitic stainless steelenvelopes are of a foil thickness of not greater than about .006 inch.

References Cited RICHARD J. HERBST, Primary Examiner E. M. COMBS,Assistant Examiner US. (:1. X.R. 72 47; 14s 11,5

